If $x=\frac{1}{t^2}$ and $y=\frac{1}{t^3}$, then $\frac{d^2y}{dx^2}$ at t = 1 is :

$\frac{3}{2}$ $\frac{1}{2}$ $\frac{2}{3}$ $\frac{3}{4}$

$\frac{3}{4}$